Apparatus and method for removing oil from oil-coated particles

ABSTRACT

Particles of sand coated with oil are loaded into a housing (1). A fluidizing unit (3) is provided towards the bottom of the housing. The fluidizing unit fluidizes the particles and discharges them from the housing to a separator, such as one or more hydrocyclones (8, 14), in which the oil and sand particles are separated. The coated particles can be introduced in the housing (1) via a cyclonic separator (50, 52) which provides some preliminary separation.

BACKGROUND OF THE INVENTION

The production fluid from an oil well includes varying proportions ofoil, water and gas in which are entrained solid particles, hereinafterreferred to as "sand". This mixture is normally fed to a phase separatorin which settling under gravity occurs into an upper gaseous layer, amiddle oil layer, and an lower water layer. These are removed throughseparate outlets from the separator. The sand naturally settles out inthe bottom of the water layer in the tank and it will be undesirable toallow this to be discharged through the water outlet, not least becausethe sand particles will be coated with oil and it is unacceptable todischarge such coated particles with the water back into theenvironment. Consequently the coated sand particles are recovered fromthe bottom of the phase separator and, according to one method, suppliedto a vessel fitted with impellers which rotate in opposite directions tosuspend the contaminated particles in water, causing dynamic contact ofthe particles, which mechanically strips away the oil coatings from thesolid particles. The clean solid particles may then be discharged to theenvironment but the carrier water has to be treated, normally byflotation, to remove the oil. This is difficult, costly, and takes spacewhich is at a premium on, for example, a marine platform.

Other situations obtain where it is necessary to clean oil-coated sandparticles, for example when an oil spillage contaminates a beach.

SUMMARY OF THE INVENTION

According to the present invention, a method for separating oil fromparticles coated with oil comprises loading the coated particles into ahousing containing a fluidising unit which has a liquid supply duct withan outlet and arranged to be fed with water under pressure from outsidethe housing, and a discharge duct within the liquid supply duct havingat its end an inlet projecting beyond the outlet of the liquid supplyduct, the discharge duct leading to a separator; feeding water to theliquid supply duct and causing it to swirl at the outlet such that itdisturbs the oil and sand particles to cause the oil to be stripped, atleast partially, from the particles, and causes the oil and sandparticles entrained in the water to travel into the discharge duct andhence to a separator, where the oil, water and solid particles undergoseparation.

The fluidising unit creates a swirling vortex which violently strips oilfrom the sand particles and also discharges the oil, water and sandwithout the need for any moving parts in contact with the separatedcomponents.

Although the method may be used for example, for cleaning sand from acontaminated beach after an oil spill, or for cleaning drilling mud, sothat the mud is clean enough to be dumped, it is particularly useful fortreating the oil-coated sand sedimented in a three phase separator forseparating the components of the production fluid from an oil well.Alternatively, the contaminated sand in the production fluid may betreated by the method after settlement of the contaminated sand from theproduction fluid prior to its entry into the three phase separator. Inthese cases, the water fed to the liquid supply duct of the fluidisingunit is preferably taken from the water outlet of the phase separator.This has the advantage that the water will still be at a comparativelyhigh temperature, which will promote the stripping of the oil coatingfrom the sand particles.

The separator may include a centrifuge or a single hydrocyclone stagewhich is arranged such that the oil is substantially separated andreports to the overflow and the water and sand report to the underflow.However, to improve the separation, two hydrocyclone stages arepreferably provided, the first of which is a liquid/solid separationhydrocyclone, which substantially separates sand, which reports to theunderflow from oil and water which report to the overflow, and thesecond of which is a liquid/liquid separation hydrocyclone, whichsubstantially separates water which reports to the underflow from oilwhich reports to the overflow. The use of hydrocyclones generatessufficient centrifugal forces to cause further scrubbing of the oil fromthe sand particles.

The efficiency of solid/liquid separation hydrocyclones is criticallydependent upon the density of slurry fed to the hydrocyclone inlet. Thepresent invention, utilising the special fluidising unit, isparticularly suitable for maximising the efficiency of a solid/liquidseparation hydrocyclone in the separator because the density of theslurry discharged through the discharge duct of the fluidising unit canbe accurately controlled by fine tuning the parameters of the fluidisingunit. The particular parameters which are either preset for a particularsystem, or are adjustable to accommodate changing conditions are thepressure/flow of the water fed to the liquid supply duct of thefluidising unit, and the axial separation of the outlet of the liquidsupply duct and the inlet of the discharge duct of the fluidising unit.

One aspect of the invention includes an apparatus for carrying out themethod according to the present invention, the apparatus comprising ahousing having an inlet for oil-coated sand particles, the housingcontaining a fluidising unit which has a liquid supply duct with anoutlet and arranged to be fed with water under pressure from outside thehousing, and which has a discharge duct within the liquid supply ductand having at its end an inlet projecting beyond the outlet of theliquid supply duct, the discharge duct leading to a separator comprisinga liquid/solid separation hydrocyclone, the overflow outlet of whichleads to a liquid/liquid separation hydrocyclone.

A further aspect of the invention includes an apparatus for carrying outthe method according to the present invention, the apparatus comprisinga housing having an inlet for oil-coated sand particles, the housingcontaining a fluidising unit which has a liquid supply duct with anoutlet and arranged to be fed with water under pressure from outside thehousing, and which has a discharge duct within the liquid supply ductand having at its end an inlet projecting beyond the outlet of theliquid supply duct, the discharge duct leading to a separator; and athree phase separator, to which an overflow from the housing leads. Thisapparatus is suitable for use with the method which treats theproduction fluid prior to its entry into the three phase separator. Theseparator may be a centrifuge, or a solid/liquid hydrocyclone, theoverflow of which leads to the three phase separator. Water from thethree phase separator may be supplied to the liquid supply duct of thefluidising unit. In order to improve the settlement of sand within thehousing, the inlet for oil-coated sand particles preferably leads to acyclonic separator, the underflow of which discharges sand with someassociated fluids to a lower portion of the housing, and the overflow ofwhich discharges oil and water, substantially free of sand, to an upperpart of the housing.

This arrangement forms an independent aspect of the present inventionwhich may be defined as a separator for separating solid particles froma mixture containing solid particles and a fluid component, such as aproduction fluid containing sand particles, the separator comprising ahousing having an inlet for the mixture and a separated fluid outletassociated with an upper part of the housing; the inlet for the mixtureleading to a cyclonic separator such that the mixture is caused to swirlin the cyclonic separator, the cyclonic separator having an overflow forthe discharge of fluids to an upper part of the housing, and anunderflow for the discharge of solid particles and some fluid to a lowerpart of the housing; and, associated with the lower part of the housing,a fluidising unit which has a liquid supply duct with an outlet andarranged to be fed with liquid under pressure from outside the housing,and which has a discharge duct within the liquid supply duct and havingat its end an inlet projecting beyond the outlet of the liquid supplyduct.

A plurality of baffles may be provided in the housing to provide atortuous path to the upper part of the housing for any fluid dischargedfrom the underflow of the cyclonic separator, and which deter any solidparticles discharged from the underflow of the cyclonic separator fromreaching the upper part of the housing. If the mixture is, for example,a production fluid which forms a gas core in the cyclonic separator,then it is advantageous to provide a gas overflow outlet positioned onthe axis of the cyclonic separator.

Alternatively the housing of the separator is formed as a pressurevessel; and the cyclonic separator is at least one hydrocyclone, theunderflow outlet or outlets of which discharge(s) into a closedunderflow chamber of the pressure vessel, in which chamber thefluidising unit is provided.

If there is a bank of more than one hydrocyclone within the pressurevessel, the inlet for the mixture to the vessel may open into an inletchamber and the overflow outlets of the hydrocyclones may open into anoverflow chamber, the inlet and overflow chambers being sealed from oneanother and from the underflow chamber.

By its nature, the hydrocyclone provides a pressure drop from its inletto its underflow and overflow outlets, the pressure drop to the overflowoutlet in a solid/liquid separating cyclone, normally being greater thanthat between the inlet and underflow chambers. Consequently, andaccording to a preferred aspect of the invention, an upper part of theunderflow chamber is connected to an overflow outlet from the vessel inorder that any gas or oil carried through the hydrocyclone underflowinto the underflow chamber flows to and is entrained by the fluidsleaving the overflow outlet for fluids from the vessel.

A further advantage of the pressure drop between the production fluidinlet pressure and the pressure obtaining in the underflow chamber maybe obtained by driving the fluidising unit with the inlet pressure, thatis by connecting the supply duct of the fluidising unit to a mixtureinlet line to the vessel so that a side stream of the mixture drives thefluidising unit. In this way a small proportion of the excess pressurein the mixture is used up in driving the fluidising unit, without theneed for any separate pressure source.

Fluidising units of a type suitable for use in any aspect of the presentinvention are disclosed in our earlier U.S. Pat. Nos. 4,978,251,4,952,099 and 4,992,006. The fluidising unit is capable of removingparticles on line, without the need to depressurise the housing.

The fluidising units of any aspect of the present invention may beorientated with the liquid supply duct and discharge duct openingdownwardly. Alternatively, the fluidising unit may be orientated withthe liquid supply duct and the discharge duct opening upwardly. In thiscase, it is preferable to close the gap between the liquid supply ductunit is not in use. The gap can be closed by a valve which may be biasedclosed and forced open by liquid in the liquid supply duct, or by theliquid supply duct being movable with respect to the discharge duct toclose the gap.

In any aspect of the present invention the swirl of the liquid at theoutlet of the liquid supply duct may be caused by inclined vanes in theliquid supply duct, and/or by the liquid supply duct having a tangentialinlet into a cylindrical chamber upstream of the liquid supply ductoutlet.

In the fluidising unit used with any aspect of the present invention, ajet pump can be provided on the discharge duct to boost the pressure inthe discharge duct. When the discharge duct leads to a cycloneseparator, the liquid supplied to the jet pump can be controlled tocontrol the liquid/solid ratio to that required by the cycloneseparator.

In any aspect of the present invention, to enhance further the scrubbingof the oil from the sand particles, chemicals may be added to the liquidwhich is fed to the liquid supply duct of the fluidising unit.

In any aspect of the present invention, in order to accommodate varyingsand loads, the amount of sand in the housing may be sensed, for exampleusing a vibrating probe to sense the level of sand or a load cell tosense the mass of sand, so that the fluidising unit can be operated whenthe amount of the sand within the housing reaches a threshold value.

By removing abrasive particles by any aspect of the is presentinvention, expensive upstream equipment and valves are protected fromabrasive damage.

BRIEF DESCRIPTION OF THE DRAWING

Examples of systems employing the method of the present invention willnow be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a first system;

FIG. 2 is a schematic diagram of a system in which the first system isused for de-oiling sand from a phase separator;

FIG. 3 is a schematic diagram of a system in which a modified version ofthe first system is used for de-oiling sand to a phase separator;

FIG. 4 is an axial section of a housing including a sand trap which is amodified version of that shown in FIG. 3; and

FIG. 5 is an axial section of a housing including a sand trap which is amodified version of that shown in FIG. 3.

In FIG. 1, a housing 1 has an inlet 2 for sand contaminated with oil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fluidising unit 3 is provided within the housing 1. The unit has aliquid outlet 4 which is fed with water by a liquid supply duct 5. Theliquid supply duct 5 is provided with means such as inclined vanes (notshown) for swirling the liquid discharged from the liquid outlet 4. Adischarge duct 6 is located within and is coaxial with the liquid supplyduct 5. The discharge duct 6 terminates in an inlet 7 which projectsbeyond the liquid outlet 4.

The discharge duct 6 leads to a first hydrocyclone stage 8 which isarranged to separate sand at its underflow 9 from oil and water at itsoverflow 10. The hydrocyclone stage 8 has one or more solid/liquidhydrocyclones typically constructed of polypropylene or a ceramicmaterial. The sand from the underflow 9 is discharged into a container11 from which it can be discarded through a nozzle 12. Excess water canbe drained through a second nozzle 13. The oil and water from theoverflow 10 of the first hydrocyclone 8 are fed to a second hydrocyclonestage 14 where the water at the underflow 15 is separated from the oilat the overflow 16. The second hydrocyclone stage 14 has one or moreliquid/liquid hydrocyclones of the type described in GB 2221408. Thehydrocyclones 8,14 may be fitted with ceramic components at points ofhigh erosion potential.

The oil from overflow 16 will still contain some water. It is thereforefed to a tank 17 having first 18 and second 19 chambers. The firstchamber has a water discharge line 20 the flow through which iscontrolled by a valve 21. The second chamber 19 has an oil dischargeline 22. An overflow line 23 leads from the housing 1 to the firstchamber 18 of the tank 17.

A water recycling system 24 is provided with a pump 25 which receiveswater from nozzle 13, underflow 15 and water discharge line 20 and pumpsit to the liquid supply duct of the fluidising unit 3.

In use, the sand particles contaminated with oil are loaded into thehousing 1 through the inlet 2. Once there is sufficient contaminatedsand in the housing 1, the fluidising unit 3 is activated. This involvessupplying water under pressure to the liquid supply duct 5. The water iscaused to swirl as it leaves the liquid outlet 4 thereby generating aprecessing vortex core directly under the inlet 7 of the discharge duct6. The precessing vortex core causes violent pulsating forces which bothfluidise and mix the contaminated sand in the zone of influence of thecore. The mixing action causes the sand particles to contact each otherwith sufficient energy to scrub away some or all of the oil coating fromthe sand. Typically, about half of the water fed to the fluidising unit3 is discharged through the discharge duct 6 with the sand particleswhile the rest of the water remains in the housing 1 in place of thedischarge sand. The addition of appropriate chemicals to the liquidsupply duct 5 facilitates this scrubbing action. The precessing vortexcore causes the sand and oil entrained in the water to be dischargedfrom the housing through discharge duct 6.

The oil, sand and water from the discharge duct 6 encounter the firsthydrocyclone stage 8 where the sand is separated from the oil and waterand reports to the underflow 9. The oil and water report to the overflow10 and encounter the second hydrocyclone stage 14 wherein the waterreports to the underflow 15 and the oil is discharged through overflow16 to the first chamber 18 of tank 17. The valve 21 remains closed untilthe fluids in the first chamber 18 reach a level to allow the upperlayer of oil to flow into the second chamber 19. The valve 21 can beoperated automatically using a level control which detects the positionof the water-oil interface within the first chamber 18. The oil in thesecond chamber 19 is discharged through line 22 for recovery. The waterfrom the first chamber 18 is discharged through line 20 on opening ofthe valve 21 and may be fed to the liquid supply duct 5 of thefluidising unit 3 through water recycling system 24, driven by pump 25.

FIG. 2 shows the system of FIG. 1 being used to de-oil sand extractedfrom a three phase separator 26. Such a phase separator may be used toseparate fluids from an oil well which are contaminated with sand. Thesand will tend to settle to the bottom of the separator 26 therebyreducing the effective volume and clogging the ports of the phaseseparator. The sand which is removed will be contaminated with oil.

The phase separator 26 has conventional gas 27, oil 28 and water 29outlets. In addition, an outlet 30 leading to housing 1 is provided forthe contaminated sand which has settled on the bottom of the tank. Awater inlet 31 which allows water into the phase separator 26 in orderto flush out the sand and oil through outlet 30 is connected to a pump32.

A load cell 33 detects the mass of the contents of housing 1 and outputsa signal to a controller 34 which gives an indication of the amount ofsand in the housing 1. The load cell 33 could equally be replaced by avibrating probe sensor within the housing 1 to detect the level of sandwithin the housing 1.

The controller 34 receives additional signals from first 35 and second36 flow sensors, which detect abnormal flow conditions in the system,and sends signals to a plurality of valves, the pump 32 and a chemicaldosing unit 37 in a manner to be described. The remainder of the systemis as described with reference to FIG. 1.

The size of the fluidising unit used and the length of time for which itis run depends on the content of sand within the oil. For example, in anoil field where the sand load is light (less than 0.5 tonnes/day), afluidising unit having an inlet 7 with a 25 mm bore may typically be runfor about two hours a week. Where the sand load is medium (up to 20tonnes/day) the same fluidising unit is typically run for twelve hours aday. Where the sand load is heavy (greater than 20 tonnes/day) afluidising unit in which the inlet 7 has a 50 or 75 mm bore might be runfor up to twelve hours a day.

In order to remove the sand from the phase separator 26, the pump is runat a first speed to pump water through the line 31 to flush out thecontaminated sand from the bottom of the phase separator 26 throughoutlet 30 and into the housing 1. The heavy sand sinks to the bottom ofthe housing 1, while the oily water overflows through the overflow line23 to a slop tank, or for further treatment. As the housing 1 fills withsand and the oily water is displaced, the overall mass increases. Theincreasing mass is sensed by the load cell 33 or the increasing level ofsand is sensed by a vibrating probe, and, when the amount reaches athreshold value, the controller 34 shuts off the supply of water fromthe pump 32 to the phase separator 26 so that the flushing operation isterminated. The pump 32 is then run at a second speed which is lowerthan the first speed to pump water to the liquid supply duct 5 of thefluidising unit 3. This causes the contaminated sand to be scrubbed andremoved from the housing 1 and separated as described with reference toFIG. 1. If the sand load is unusually light, the amount may not reachthe threshold value, in which case, the flushing operation will beterminated by a timer.

As the fluidising unit 3 discharges the sand from the housing 1, theamount of sand within the housing will fall until a value is detectedindicating that there is no sand present within the housing, at whichpoint the system will be shut down until the cleansing of the separator26 is required again.

In place of the double hydrocyclone stage 8,14 shown in FIG. 2. It ispossible to use a single hydrocyclone stage in which the oil reports tothe overflow and the water and sand report to the underflow.

FIG. 3 shows a system for removing the bulk of the sand, for example,particles having a diameter of more than 0.1 mm, from the productionfluid of an oil well before it enters a three phase separator. Thissystem includes a system similar to that shown in FIG. 1 although it isonly shown with a single hydrocyclone stage, the overflow of which leadsto the inlet of the three phase separator instead of the tank.

The production fluid from the oil well enters a sand trap 38 in thehousing 1. The contaminated sand settles to the bottom of the housing,while the liquid part of the production fluid overflows from the housing1 through overflow 23 which leads to the inlet of the three phaseseparator 26. A detector 39 detects when the level of sand in thehousing is at a predetermined level and the fluidising unit 3 isoperated in the same manner as described with reference to FIG. 1. Thedischarge duct 6 of the fluidising unit 3 leads to a first hydrocyclone40 which separates the sand and water, which report to the underflow 41from the oil and some water which report to the overflow 42. The cleansand and water from the underflow 41 encounter a degas stage 43 beforebeing dumped. The oil and water from overflow 42 are pumped back to theinlet of the three phase separator 26.

Water taken from the water outlet 29 of the three phase separator 26 isfed to liquid/liquid hydrocyclone 44, and the cleaned water from theunderflow 45 is fed to the liquid supply duct, after being supplied withthe appropriate chemicals from chemical dosing unit 37. If thefluidising unit 3 is not being run, the operation of appropriate valvesensures that the water from the underflow 45 is not supplied to theliquid supply duct 5, but is instead dumped after encountering degasstage 46. Oil containing a water component is discharged from the outlet47 of the hydrocyclone 44 and is pumped to the inlet of the three phaseseparator 26.

A second fluidising unit 48 which operates in the same way as thefluidising unit 3, previously described, is provided in a trap 49 in thebottom of the three phase separator 26. This unit 48 operates in thesame way as the fluidising unit 3 previously described, and is used toclean the three phase separator 26 during normal shut downs, or, if forsome reason sand is carried over into the three phase separator 26 fromthe housing 1. The contaminated sand removed by the second fluidisingunit 48 is dealt with in the same way as that from the first fluidisingunit 3.

A modified version of the sand trap 38 is illustrated in FIG. 4. Thisshows a housing 1 having a fluidising unit 3 and an overflow line 23 asshown in the previous examples.

The inlet 2 for production fluid leads tangentially to a cyclonicseparator 50 such that the production fluid is caused to swirl. Thecyclonic separator 50 has an overflow 51 for oil, gas and some water. Asecond overflow 52 is provided for gas which forms in core at the axisof the cyclonic separator 50. A choke valve 53 controls the outlet ofthe gas. This gas will be quite wet and will therefore require a gasdehydration device to remove liquids prior to the gas being set toflare. Any liquids removed are sent to the three-phase separator 26.

The underflow 54 of the cyclonic separator discharges the majority ofthe sand together with some water, oil and gas towards a lower part ofthe housing 1. A baffle 55 distributes the sand, which is thrownoutwards by the swirling action in the cyclonic separator 50, around thelower part of the housing 1. A series of further baffles 56 create atortuous path through which any water, oil or gas from the underflow 54flows back to the top part of the housing 1. This tortuous path detersthe sand from migrating to the top of the housing.

As shown in FIG. 5 a pressure vessel 1 has an inlet 2 for productionfluid, leading into an inlet chamber 57 which is sealed by walls 58 and.59 from an overflow chamber 60 having an outlet 23, and an underflowchamber 61. Supported by the walls 58 and 59 are a bank of hydrocyclones62, of which only two are shown. Each has its inlet 63 open to thechamber 57, its overflow outlet 64 discharging into the chamber 60, andits underflow outlet 65 discharging into the chamber 61. Thehydrocyclones 62 are designed primarily to separate sand from fluid, themajority of the fluid passing into the overflow chamber 60 and out ofthe outlet 23 whilst the majority of the sand with some fluid, includingwater, oil and gas passes into the underflow chamber 61 where the sandsediments in the bottom of the chamber 61. From here it may becontinuously or intermittently discharged by means of a fluidising unit3 similar to that shown in FIG. 4.

The primary production fluid supplied to the inlet 2 is via a main line66. A branch 67 is taken off this line and is connected to the liquidsupply duct 5 via a control valve 68 to drive the fluidising unit.

Inevitably some gas and oil will collect in the top of the underflowchamber 61 and as this chamber will be at a somewhat higher pressurethan obtaining in the outlet chamber 60 and outlet 23, it may be removedvia an outlet 69 connected via a line 70 to the main outlet line 71leaving the outlet 23. In this way the chamber 61 can be purged oflighter fluids, heavier fluid, particularly water, being dischargedthrough the discharge duct 7 from the fluidising unit 3.

I claim:
 1. A method for removing oil from particles coated with oil,the method comprising the steps of:loading a slurry of coated particlesinto a housing (1) containing a fluidizing unit which has an axis, aliquid supply duct (5) with an outlet (4), and a discharge duct (6)extending through said liquid supply duct so that said liquid supplyduct surrounds a portion of said discharge duct, the liquid supply ducthaving at its end an inlet (7), the outlet of the liquid supply duct andthe inlet of the discharge duct being axially separated in thefluidizing unit; selecting the amount of axial separation of the liquidsupply duct outlet and the inlet of the discharge duct in accordancewith a desired slurry density for a discharge of the fluidizing unit;selecting flow characteristics for water fed to the liquid supply ductin accordance with a desired slurry density for a discharge of thefluidizing unit; and feeding water to the liquid supply duct underpressure from outside the housing in accordance with the selected flowcharacteristics and causing it to swirl at the outlet to create a vortexwhich disturbs the oil and particles to cause the oil to be stripped, atleast partially, from the particles loaded in the housing, and causesthe oil and particles entrained in the water to travel into thedischarge duct as a discharge slurry of the desired density, the slurrybeing comprised of the solid particles, the oil stripped from theparticles, and the water; and supplying the discharge slurry to aseparator where the oil, water, and solid particles undergo separation,the separation efficiency of the separator being slurry densitydependent and the selection of the axial separation and flowcharacteristics being such as to establish the discharge slurry densityat that which provides a desired separation efficiency of the separator.2. A method according to claim 1, wherein the selecting steps arefurther defined as selecting the outlet-inlet axial separation and flowcharacteristics to establish the discharge slurry density at that whichmaximizes the separation efficiency of the separator.
 3. A methodaccording to claim 1, wherein the supplying step is further defined assupplying the discharge slurry to a liquid/solid hydrocyclone separator(40) which substantially separates sand and water which report to anunderflow (41) from oil and water which report to an overflow (42).
 4. Amethod according to claim 1, wherein the step of supplying the dischargeslurry to a separator is further defined as supplying the dischargeslurry to a separator comprised of two hydrocyclone stages (8, 14), thefirst of which is a liquid/solid separation hydrocyclone stage (8) whichsubstantially separates sand, which reports to an underflow (9) from oiland water which report to an overflow, and the second of which is aliquid/liquid separation hydrocyclone stage (14), which substantiallyseparates water which reports to an underflow (15) from oil whichreports to an overflow (16).
 5. A method according to claim 1, whereinthe step of causing the water to swirl at the outlet of the liquidsupply duct is further defined as deflecting the water by inclined vanesin the liquid supply duct.
 6. A method according to claim 1, wherein thestep of causing the water to swirl at the outlet of the liquid supplyduct is further defined as tangentially feeding the water into acylindrical chamber of the fluidizing unit upstream of the liquid supplyduct outlet.
 7. A method according to claim 1, further defined asincluding the step of adding chemicals to the water which is fed to theliquid supply duct of the fluidizing unit.
 8. A method according toclaim 1, further defined as sensing the amount of particles contained inthe housing and operating the fluidizing unit when the amount ofparticles reaches a threshold value.
 9. An apparatus for removing oilfrom particles coated with oil, said apparatus comprising:a housing (1)for receiving a slurry of coated particles, said housing containing afluidizing unit (3) which has an axis, a liquid supply duct (5) with anoutlet (4), and a discharge duct (6) extending through said liquidsupply duct so that said liquid supply duct surrounds a portion of saiddischarge duct, the liquid supply duct having at its end an inlet (7),the outlet of the liquid supply duct and the inlet of the discharge ductbeing axially separated in the fluidizing unit by an amount selected inaccordance with a desired slurry density for a discharge of thefluidizing unit, water being fed to the liquid supply duct underpressure from outside the housing having flow characteristics selectedin accordance with a desired slurry density for a discharge of thefluidizing unit, the water swirling from the liquid supply duct outletto create a vortex in the housing which disturbs the oil and particlesto cause the oil to be stripped, at least partially, from the particlesreceived in the housing, and causes the oil and particles entrained inthe water to travel into the discharge duct as a discharge slurry of thedesired density, the slurry being comprised of the solid particles, theoil stripped from the particles, and the water; and a first separator(8) coupled to said discharge duct of said fluidizing unit for receivingthe discharge slurry, said first separator comprising a liquid/solidseparation hydrocyclone having an overflow outlet; and a secondseparator (14) coupled to said overflow outlet of said first separator,said second separator comprising a liquid/liquid separationhydrocyclone, the separation efficiency of at least one of saidhydrocyclones being slurry density dependent.
 10. An apparatus accordingto claim 9, wherein a closable gap exists between the liquid supply duct(5) and the discharge duct (6), said gap being closed when saidfluidizing unit (3) is not in use.
 11. An apparatus according to claim9, further including a pump coupled to said discharge duct for boostingthe pressure in said discharge duct (6).
 12. An apparatus according toclaim 9, wherein said inlet (7) of said discharge duct (6) projectsbeyond said outlet (4) of said liquid supply duct (5).
 13. An apparatusfor removing oil from particles coated with oil, said apparatuscomprising:a housing (1) for receiving a slurry of coated particles,said housing containing a fluidizing unit (3) which has an axis, aliquid supply duct (5) with an outlet (4), and a discharge duct (6)extending through said liquid supply duct so that said liquid supplyduct surrounds a portion of said discharge duct, the liquid supply ducthaving at its end an inlet (7), water being fed to the liquid supplyduct under pressure from outside the housing, the outlet of the liquidsupply duct and the inlet of the discharge duct being axially separatedin the fluidizing unit by an amount selected in accordance with adesired slurry density for a discharge of the fluidizing unit, waterbeing fed to the liquid supply duct under pressure from outside thehousing having flow characteristics selected in accordance with adesired slurry density for a discharge of the fluidizing unit, the waterswirling from the liquid supply duct outlet to create a vortex in thehousing which disturbs the oil and particles to cause the oil to bestripped, at least partially, from the particles received in thehousing, and causes the oil and particles entrained in the water totravel into the discharge duct as a discharge slurry, the slurry beingcomprised of the solid particles, the oil stripped from the particles,and the water, said housing having an overflow conduit (23); and a firstseparator (40) coupled to said discharge duct of said fluidizing unitfor receiving the discharge slurry and separating the particles from theoil and water, the separation efficiency of said first separator beingslurry density dependent; and a second, three phase, separator (26)receiving the oil and water from said first separator unit, the overflowconduit of said housing being connected to said second separator.
 14. Anapparatus according to claim 13, wherein said second separator isconnected to said liquid supply duct for supplying liquid to said liquidsupply duct.
 15. An apparatus according to claim 13, wherein saidhousing has an upper portion and a lower portion and an inlet forreceiving the slurry of coated particles, and wherein said housingincludes a cyclonic separator (50, 62) coupled to said inlet, saidcyclonic separator having an underflow (54, 65) which dischargesparticles and a quantity of liquid to the lower portion of said housingand an overflow (51, 64) which discharges oil and water, substantiallyfree of sand, to the upper portion of said housing.
 16. A separator forseparating solid particles from a mixture containing solid particles anda fluid component, said separator comprising:a housing (1) having anupper part and a lower part, said housing having an inlet (2) for themixture and a fluid outlet (23) associated with said upper part; theinlet for the mixture leading to a cyclonic separator (50, 62) such thatthe mixture supplied through the inlet is caused to swirl in thecyclonic separator, said cyclonic separator having an overflow (51, 64)for the discharge of fluids to said upper part of said housing, and anunderflow (54, 65) for the discharge of solid particles and a portion offluid to said lower part of said housing; and a fluidizing unit (3)positioned in said lower part of said housing, said fluidizing unithaving a liquid supply duct (5) with an outlet (4) and arranged to befed with liquid under pressure from outside the housing, and having adischarge duct (6) extending through said liquid supply duct so thatsaid liquid supply duct surrounds a portion of said discharge duct, theliquid supply duct and having at its end an inlet (7) for receiving thesolid particles and discharging them from the lower part of the housing.17. A separator according to claim 16, wherein a plurality of baffles(55, 56) are provided in the housing (1) to provide a tortuous path tothe upper part of the housing for any fluid discharged from theunderflow (54) of the cyclonic separator (50), and to deter any solidparticles discharged from the underflow of the cyclonic separator fromreaching the upper part of the housing.
 18. A separator according toclaim 16, wherein said cyclonic separator (50) has an axis, about whichsaid mixture swirls and wherein said cyclonic separator has a gasoverflow outlet (52) provided on said axis.
 19. A separator according toclaim 16, wherein the housing (1) is a pressure vessel, and wherein thecyclonic separator comprises at least one hydrocyclone (62), theunderflow outlet (65) of which discharges into a closed underflowchamber (61) of the pressure vessel, in which chamber the fluidizingunit (3) is provided.
 20. A separator according to claim 19, furtherincluding a plurality of hydrocyclones (62), the fluid inlet (2) for themixture opening into an inlet chamber coupled to inlets of saidhydrocyclones, the overflow outlets (64) of said hydrocyclones openinginto an overflow chamber (60), said inlet chamber and said overflowchamber being sealed from one another and from said underflow chamber(61).
 21. A separator according to claim 19, wherein an upper part ofsaid underflow chamber (61) is connected to an overflow outlet (69) fromsaid housing (1), said overflow outlet being connected to said fluidoutlet (23) so that the portion of fluid discharged through thehydrocyclone underflow outlet (65) into said underflow chamber (61) maybe conveyed to said separated fluid outlet (23).
 22. A separatoraccording to claim 19, wherein said liquid supply duct (5) is connectedto said inlet (2) for said housing for receiving liquid for supply tosaid liquid supply duct (5).
 23. A separator according to claim 16,wherein said fluidizing unit is further defined as including means forcausing the liquid leaving said liquid supply conduit (5) to swirl. 24.A separator according to claim 23, wherein said means comprises inclinedvanes in said liquid supply duct (5).
 25. A separator according to claim23, wherein said fluidizing unit has a cylindrical chamber upstream ofsaid liquid supply duct outlet and wherein said swirling means comprisesa tangential inlet for said liquid supply duct (5) into said cylindricalchamber.
 26. A separator according to claim 16, further including meansfor adding chemicals to the liquid fed to said liquid supply duct (5) ofsaid fluidizing unit (3).
 27. A separator according to claim 16, furtherincluding a sensor for sensing the amount of particles in said housing(1), whereby the fluidizing unit (3) can be operated when the amount ofparticles reaches a threshold value.
 28. A separator according to claim16, wherein a closable gap exists between said liquid supply duct (5)and said discharge duct (6), said gap being closed when said fluidizingunit (3) is not in use.
 29. A separator according to claim 16, furtherincluding a pump coupled to said discharge duct (6) for boosting thepressure therein.
 30. A separator according to claim 16, wherein saidinlet (7) of said discharge duct (6) projects beyond said outlet (4) ofsaid liquid supply duct (5).